It is known in the art to provide a diesel engine with exhaust filter means comprising a particulate trap to collect particulates from the exhaust gas stream during the engine operation. Such particulates consist largely of carbon particles which, with continued operation, tend to plug the exhaust filter, causing a restriction to normal exhaust gas flow. Such a restriction may be avoided by periodically cleaning the particulates from the exhaust gas filter.
The cleaning of particulates from an exhaust filter may be accomplished by any practical method. One suggested method is incineration in place, brought about by increasing the engine exhaust gas temperature to the combustion temperature of the collected particulates. This may be accomplished, for example, by heating the exhaust gas stream in the manner disclosed in the co-pending United States patent application Ser. No. 872,360 filed Jan. 26, 1978, now U.S. Pat. No. 4,167,852, in the name of Otto A. Ludecke and assigned to the assignee of the present invention.
Under some circumstances, the normal operating temperature of the diesel engine exhaust gases may be sufficiently high to incinerate collected particulates in an appropriately located exhaust filter. For example, U.S. Pat. No. 3,937,015 Adako et al discloses a particular construction of exhaust manifold mounted filter which, it is asserted, will be cleaned by the burning of collected carbon particles caused by high exhaust gas temperatures reached during heavy load operation of the engine. However, such a system does not provide control of the frequency or extent of the burn periods nor of the maximum temperatures reached which, if these are excessive, may shorten the life of or permanently damage the exhaust filter element.
Also, in some engine applications, particularly those for use in lighter automotive vehicles, operation under heavy load may rarely occur. In this case, the engine exhaust temperatures may not be sufficient to burn off collected particulates at the time such action is desirable. For example, in a currently produced automotive diesel engine operated in simulated city driving conditions at 25 miles per hour road load, the exhaust manifold temperature was measured at only 360.degree. F., which is far below the temperature of approximately 900.degree. F. required to incinerate trapped particulates from a filter device in the exhaust. Thus, in order to adequately control the periods and temperatures of incineration, we consider it desirable to provide controllable means for increasing the exhaust gas temperature to accomplish the desired periodic burn off of particulates.